1. Field of the Invention
The invention relates to internal bone fixation plates which are used by surgeons to hold together broken bones so as to facilitate healing of the bone.
2. Description of the Prior Art
The development of implantable plates for internal fixation of fractured long bones was first introduced late in the nineteenth century. The basic concept is to place a plate in contact with the bone so that the plate spans the fracture, and to fasten the plate to the bone on both sides of the fracture by means of screws. The plate and the screws must, of course, be made of materials that will not cause adverse reactions in the body and that will not deteriorate in any reasonable time. Internal fixation plates are clinically appealing because they produce the significant advantages of a rapid return of functional weight bearing, improved rehabilitation of surrounding soft tissues, and shortened hospital stay.
Fundamental changes in the concept of fracture fixation took place in 1946 with the introduction of compression plates. These plates load the bone in compression at the time of fixation. Such plates are often very rigid, and rigidity deprives the bone of normal stresses. The lack of stresses results in loss of bone mass and local weakening of the cortex of the bone, which is the bone's outer wall. It also restricts load-induced deformations at the fracture site, which inhibits the healing process by restricting the exchange of liquids via the canaliculi. The exchange of liquids is important for the nutrition of the osteocytes.
An important unfavourable effect of rigid fixation is the suppression of the osteogenic potential of the periosteum during the healing phase, so that limited or no external callus develops around the fracture. This makes radiologic assessment of the state of union of a fracture impossible. It also delays the process of union, since the healing has to rely mostly on the direct growth of the Haversian envelope across the fracture. During the phase of remodelling, the fact that the plate bears most of the load leads the loss of beneficial structural alignment of newly-formed osteons and lamellae, and thus to a weak union.
Delayed union and loss of bone mass and structure have led to refractures after removal of the plate. No objective criteria exist to define the best time to remove these devices, which are designed to fulfil a temporary function. Too-early removal might cause a refracture due to incomplete healing, and too-late removal might cause a refracture due to weakening of the bone under the plate.
Various plates to hold the broken bone in a good position for healing while reducing the rigidity of fixation and the shielding from stress have been tried and are well known in the art. The previous method of achieving this goal was by making the plates of a material having a lower modulus of elasticity, or by reducing their cross section. However, reduction of plate rigidity by such methods affects the axial, bending and torsional stiffnesses, and this is in opposition to the basic immediate need of holding the broken portions of the bone in their desired relative position while also permitting the bone to endure axial stress. The dilemma in the design of bone fixation plates is in the need to maintain very great stability of the relative positions of the broken pieces of bone, for which high bending and torsional stiffnesses are needed, while at the same time to allow axial loading of the bone, for which low axial stiffness is needed.